Methods We (1) retrospectively studied the relationship between TSH and FT4 in a large unselected clinical sample (n=6641) of primary hypothyroid, euthyroid and hyperthyroid subjects, and (2) applied
a mathematical model of thyroid hormone feedback control to assess the relation between structural parameters and TSH levels
in the different functional states.

Results When separately analysing total sample and untreated subjects, the correlation slope for logTSH versus FT4 for hypothyroid subjects was significantly different from that of the euthyroid panel and hyperthyroid subjects (the latter
being compromised by reaching the TSH assay's lower detection limit). As trends between functional states changed, each functional
segment appeared to become differently regulated. Theoretical modelling and sensitivity analysis revealed that the influence
of various structural parameters on TSH levels also depends on the overall function of the feedback loop.

Conclusions Our data suggest that the states of hypothyroidism, euthyroidism and hyperthyroidism can be regarded as differently regulated
entities. The apparent complexity could be replicated by mathematical modelling suggesting a hierarchical type of feedback
regulation involving patterns of operative mechanisms unique to each condition. For clinical purposes and assay evaluation,
neither the standard model relating logTSH with FT4, nor an alternative model based on non-competitive inhibition can be reliably represented by a single correlation comparing
all samples for both hormones in one all-inclusive group.